I'd ask your instructor, a female instructor or more experienced student, or whoever is running the showcase for advice, but in general I'd look for something that shows off the movement I'm making (hip motion, turns, etc.) without distracting from or interfering with movement. That means a top that is fairly close-fitting and a skirt in a cut and fabric that moves smoothly. A lot of people like skirts cut on the diagonal or with irregular hems. I'd go for a knee-length or shorter skirt, depending on how much leg I felt like showing but make sure it doesn't fly up too much when you turn! If there's ANY possibility of too much showing when you skirt twirls or you do a fancy move make sure you are wearing dance briefs in black or a color matching your dress (not nude). Good luck!